This week I spoke with Eduardo, a NanoDTC student who has just finished his PhD thesis on solar cells for space technologies, while working in Louise Hirst’s group. He’s now just begun his postdoc here at Cambridge and is always about the Maxwell Centre if you’re ever up for a chat! Hello Eduardo!
Can you tell me a bit about yourself and how you found yourself in Cambridge?
Yes of course! I'm from Mexico, in a city called Puebla. Most people here have never heard of it before, but it’s actually a reasonably big city. I did my undergraduate degree there in nanotechnology and molecular engineering, which I’ll admit was a bit of a niche programme! I always knew that I liked science, and when I was choosing a degree programme, I was wondering should I do chemistry? Should I do physics? Should I do biology?
When I was looking at the programs at the ‘Universidad de las Americas Puebla’ in Puebla , one of them was nanotechnology. I saw that it had a little bit of everything, a little bit of chemistry, physics, bio, etc. So, I thought, why not!
That sounds like a good decision! Did you do any internships throughout your undergraduate degree?
During the summer of my final year, I did an internship in Switzerland at EPFL, working on a project on using exosomes and gold nanoparticles for cancer detection. Back then, I thought I wanted to maybe do something focusing on nanomedicine or bio nanotechnology. And well, doing that, I realized that I didn't really want to go into this field.
So after doing the internship and finishing my degree, I still didn't know what I wanted to do! I knew that I wanted to do a masters and also a PhD, but I wasn't quite sure where I wanted to go or what I wanted to focus on. I thought the NanoDTC here at Cambridge was perfect because it gave me time during the first year to choose what I wanted to focus on during my PhD.
That really shows how important and helpful internships and research projects are! You also have to do two research projects here at the NanoDTC during the first year? Did they help you with your decision for your PhD project?
I think they did. As I’ve already mentioned, when I got here, I didn't know what I wanted to do. To be honest, I was quite surprised that I got here to begin with, but maybe that's another story!
My first project was with Rachel Oliver and Sam Stranks. I was using AFM, so measuring the topography, but also electric current on perovskite films. And the idea was to try to find correlations between maybe grain boundaries and the currents that we were getting or grain size. It was interesting. I mean, I had never used an AFM before, so it was exciting to learn how to use the AFM.
My second research project was very different, it was with Emily Ringe and John Biggins. The idea was to make liquid crystal elastomers, which are polymers, and then functionalize them with gold nanoparticles. I learned a lot because I hadn't really worked with polymers in the past, beyond learning about them in lectures. I found it challenging, but overall, it was an enjoyable experience and I gained a lot of skills.
So what helped you decide on your PhD project?
During Michaelmas and Lent term, I had lectures, just as you are having now Tara! One module we had was nanofabrication techniques, with Louise Hirst. She was talking about device fabrication, which I thought was really interesting and something I had not heard much about before coming to Cambridge.
I really enjoyed Louise’s lectures and sometimes throughout the lecture, she would talk about her research, and I thought it was interesting. I emailed Louise and told her I was interested in her work, and we decided to meet up and have a chat. During our chat, she told me all her new ideas and potential projects, which I thought sounded exciting and I would have considered doing for my PhD project.
I had to come to a decision however, and as I was choosing a PhD project, I was aware that it's not just about the project or how exciting it sounds or how many papers you could publish. It’s also about the supervisor, and I really wanted to do my PhD with someone that I could get along with. And chatting with Louise, it was very easy for me to talk to her, so I was like, this sounds like a good idea.
Cool! Can you tell me about the project you decided to work on with Louise in the end?
Yeah, of course I can! I work with solar cells, but our main interest is using those solar cells for satellites or spacecraft. The main thing about our solar cells is that they are very thin, about 80 nm. The reason why we do this is that when a solar cell is ultra-thin, they are more tolerant to radiation damage. This is important because in space there is lots of radiation, and that needs to be considered when designing our solar cell. The particles from the radiation (mainly electrons and protons with very high energies) are going to be bombarding your solar panels. So even though a solar panel could be great here on earth, once it’s up in space, it’s going to be bombarded by all those particles.
Where do those particles come from?
They mainly come from cosmic rays, solar flares and they also get trapped within radiation belts around the Earth because of the magnetic field of the Earth. So, if you want to send something up to space and you're crossing those belts, then that's a bit of an issue.
That doesn’t sound ideal!
It’s not! When you expose your solar cell to radiation, then you're introducing damage to the lattice structure, and creating defects and this limits the lifetime of your solar cell. One way we try to get about this now is covering the solar panel with a thick layer of glass, which could absorb most of the damage, and protect the solar cell.
The issue with the glass is that it is very heavy, and weight is something that is really important when you're sending things up to space. It costs thousands of dollars to send a kilogram up to space. And if you have massive solar panels and massive layers of glass, then it's quite a lot of weight and quite a lot of money to get that off the Earth. So, as always, we want to do things better. That's why we want to make our solar panels so thin. When they are this thin, they’re not very heavy and they’re also flexible. But more importantly, it's also tolerant to radiation damage. So even if you bombard that solar cell with radiation and you introduce defects to the lattice, the performance degrades much more slowly.
Well, there’s clearly a lot of advantages! What materials are these cells made out of? Silicon?
No, they’re actually not! For terrestrial applications, we normally use silicon because it's quite cheap, and we're very good now at using and fabricating silicon. But for space applications, we use fancier, more expensive materials. Normally in my group, we use gallium arsenide. This compound is much more efficient at absorbing light and in general, more tolerant to that radiation damage compared to silicon. Currently, the devices using gallium arsenide for spacecraft are very thick because current solar cells have multiple junctions, in order to be able to absorb more of the solar spectrum more efficiently and produce more power! Again, this isn’t ideal as it’s incredibly heavy! So I guess that's where my project comes in. I’m looking at the optics in the solar cells in order to make them absorb more light, even if they're very thin.
How do you make something absorb more light?
That's a good question! There are different things you can do; it’s actually called light management and there's different kind of techniques. An example of a simple technique is using an anti-reflection coating on the front surface of the cell so that you're not losing a lot of photons from reflection. You can also put a mirror at the back of the solar cell so that if you're not absorbing some photons, then you reflect them back towards the solar cell and you absorb more of them.
The coatings used are generally not very thick, maybe on the order of 100 nm or so. All these techniques are good, but they're not good enough! We want to do more, and we can do this using nanostructures. Maybe you've heard about other groups here in Cambridge who use nanostructures to confine light. We use a similar idea, using those nanostructures, but in our solar cells.
Sounds cool! But what type of nanostructures do you use?
That is kind of one of the questions that I was trying to answer in my PhD. I was mainly using arrays of nanostructures that were periodic. By being periodic, they're going to be diffracting light at different angles or diffraction orders. The solar cell is very thin, but only in the vertical direction, and it's not confined in the lateral direction. The idea is to have different diffraction orders propagating laterally in the device so that you're not losing that power. By designing these arrays of nanostructures, light can also get totally internally reflected in the solar cell so that it doesn't escape to the vacuum.
So this is another way of light management then?
Yeah, and we normally use all of the different strategies together. In the end we have the anti-reflection coating. We have the mirror. Then we also have that periodic array of nano structures.
So how do you actually go about designing the device? Is it just a trial-and-error method you use?
A lot of the work I've been doing has been on simulations because there’s a huge design space, right? There’s loads of different parameters you could change, like materials, thicknesses, the shape of the nanostructures and periodicity. I think simulations are very important to test different designs and find what you want to fabricate. Fabricating a solar cell, it's a very long process, and involves a lot of steps. Therefore, it's not really feasible to try out the entire design space experimentally. We must rely on simulations to then kind of guide your fabrication effort.
And does the performance of the device improve if we do this?
It definitely does! By adjusting all these parameters, we can get around a 50% increase in the number of photons that we're absorbing! This means that we think we could get to a 20% photovoltaic efficiency (how much of that solar energy are you converting to energy) with these very thin solar cells.
And you’re finished up now! You wrote your thesis, congratulations! What are you hoping to do after?
A few months ago, I was speaking to Louise about this, and she asked me the same question.. She proposed if I want to stay on and maybe work in the group as a postdoc for a while, and I thought it was a great opportunity so I applied for the position.
That was incredibly nice of Louise! So now that you’re finished up, do you have any inkling as to where you want to go next after the postdoc?
I’m really not sure yet! I think I'm also leaning towards industry, but I’ve also loved working here in academia here at Cambridge. But I haven’t even officially started yet because I still have my viva!
And as a final question, what advice would you give for students now picking a PhD project? Was it a good decision to do one?
I think that the PhD was a very good experience. I know it can be very busy and there's quite a lot of projects. But in my case, thankfully, it was a good experience. There’s a lot of soft skills I think you don't really notice that you gain throughout the PhD. I think I learned a lot, not only about solar cells in space, but also about how to manage my time, work with others, manage my own project and figure out what I wanted to do in the future.
I think one of the best pieces of advice would be to think about who you're going to be working with on a daily basis. It can be very tempting to just go for the most exciting project. But make sure that you enjoy working with the group and the supervisor you chose, you don't want to have a miserable life for three or four years. It’s important to us to be happy.
Thanks Eduardo, that’s definitely valuable advice and it’s great to hear about how the NanoDTC is making an impact even in space!
Special thanks to Larry Brazel for all his help :)